PSI - Issue 64

Muhammad Ishfaq et al. / Procedia Structural Integrity 64 (2024) 1540–1548 Ishfaq et al./ Structural Integrity Procedia 00 (2024) 000 – 000

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3.2. Failure Modes

Photographs of the typical failure modes of representative specimens are shown in Fig. 4. The control specimens of Group A (no stirrups) and Group B (#2@200) failed in shear with a single dominant diagonal tension crack (see Fig. 4a). The critical diagonal tension crack started in the web near the support and propagated across the shear span towards the loading point in the flange. Some secondary cracks also formed. As expected, the flexurally-strengthened specimen in Group A and B with no U-wraps (0U) also exhibited the same failure mode with approximately the same shear capacity (Fig. 4a). The strengthened specimen with 1-ply U-wrap (1U) and 2-ply U-wrap (2U) failed by debonding of U-wraps along the critical shear crack line. The critical shear crack angle value for all shear-controlled specimens varied between 30° to 40°. Complete debonding was observed in U-wraps intersected by the critical shear crack, whereas partial debonding was observed in U-wraps intersected by secondary cracks (see Fig. 4c). Group C (#4@200) specimens failed in flexural-controlled fashion. The unstrengthened control specimen failed in a ductile mode, i.e., by yielding of tension steel followed by compression failure of concrete. The flexurally strengthened specimen with no U-wraps exhibited typical IC debonding of longitudinal CFRP sheet (Fig. 4d). The specimen with 1-ply U-wrap (1U) delayed the IC debonding and failed due to the rupture of U-wraps along with debonding of longitudinal CFRP sheet (Fig. 4e). The specimen strengthened 2-ply U-wrap (2U) shifted the failure mode to partial debonding of U-wrap and concrete crushing (see Fig. 4f).

Fig. 4. Typical failure pattern of specimens after testing: (a) diagonal tension failure [0U (#2@200)]; (b) shear failure after U-wraps debonding [1U (no stirrups)]; (c) shear failure after U-wraps debonding [2U (#2@200)]; (d) IC debonding [0U (#4@200)]; (e) U-wrap rupture [1U (#4@200)]; (f) U-wrap debonding [2U (#4@200)]. 3.3. Strain in Flexural CFRP Sheets To assess the effect of U-wraps on the level of strain utilization in longitudinal CFRP sheets, it is important to examine the strain distribution and the maximum strain recorded in the constant moment region at peak load. Fig. 5 shows maximum strains recorded by strain gauges in the constant moment region of all flexurally-strengthened specimens. The ACI 440.2-prescribed debonding strain (5500 με ), manufacturer-reported ultimate strain for design, ε , (9000 με ) and measured rupture strain (13000 με ) are also provided for comparison. The specimens in Groups A (no stirrups) and B (#2@200) were shear-controlled, and the beams failed in shear before reaching the ACI 440.2 prescribed debonding strain.